The present invention relates to a solid-state color image sensor and a method of manufacturing the same, for use particularly with an electronic still camera.
A solid-state color image sensor has photoelectric conversion cells arranged in a matrix configuration and associated color filters to pick up color signals.
Various color image pickup methods have been proposed heretofore, including such as a tri-plate type method with color separation into three primaries of red, green and blue. Conventional color image pickup methods have been found not satisfactory in sensitivity. A new method solving this problem is known as a line sequential color difference signal method, which is described, for example, by Khono et al. in "Perfect Line Sequential Color Difference Signal and Mono-plate Method", The Institute of Television Engineers of Japan, Technical Report ED836 (February, 1985, and by Shin Fujimoto in "Camera System", Journal of the Institute of Television Engineers of Japan, Vol. 40, No. 11, p. 1073, 1986. With this method, color signals can be obtained line-sequentially and alternately in the form of color difference signals. This method adopts a field integration mode or a frame integration method to read image signals.
The arrangements of color filters used in the field integration mode and the frame integration mode are shown in FIG. 1.
An image sensor used in the field integration mode is constructed such that color filters of magenta (Mg) cyan (Cy), green (G) and cyan are disposed in this order in the 1st column (1), color filters of green, yellow (Ye), magenta and yellow are disposed in this order in the 2nd column (2), and the two columns with such color filter arrangement are alternately disposed over the whole area of the sensor. In order to read color signals from the sensor with an arrangement of one filter per picture cell, a mixed column readout method is used whereby at the 1st field defining a certain scan line, both the rows (a) and (b) are read out together to obtain a sum of Mg+Cy signal and G+Ye signal, and at the 2nd field defining the next scan line, both the rows (b) and (c) are read out together to obtain a sum of Cy+G signal and Ye+Mg signal. Each photoelectric conversion area is formed in alignment with the center of each color filter. The green (G) filter is obtained by superposing the yellow and cyan filters.
Although the main trend of current video cameras adopts the mixed column readout method, the image resolution thereof is dependent on the size of picture cell so that its vertical resolution is not sufficient for being used by electronic still cameras which require a high resolution.
FIG. 1 shows the arrangement of color filters used in the frame integration mode. Each color filter provided for each picture cell is constructed of two color filters each being a half size as that used in the field integration mode. At the row (a) an Mg/Cy filter and a G/Ye filter are alternately disposed, and at the (b) line a Cy/G filter and an Ye/Mg filter are alternately disposed. In reading color signals, each frame is read in unit of one line such that at the row (a) of the 1st frame, signals Mg+Cy and G+Ye are obtained and at the row (b) of the 2nd frame, signals Cy+G and Ye+Mg are obtained.
However, the material such as casein of a color filter used in the frame integration mode has a poor resolution and hence poor alignment precision at the joint between two halves of the color filter. The obtained color filter may therefore operate stronger at one color than at the other color so that flicker noises are superposed on color difference signals.